This goes in your app.php, after you register FormServiceProvider:
$app['form.type.extensions'] = $app->share($app->extend('form.type.extensions', function ($extensions) use ($app) {
    $extensions[] = new SimpleThings\FormExtraBundle\Form\Extension\FieldTypeHelpExtension();
    return $extensions;
}));
Then make your own form layout Twig, extending whatever you were using before (in this example, bootstrap_3_layout.html.twig):
{% use "bootstrap_3_layout.html.twig" %}
{% block field_help %}
    {% if help is defined %}
        <span class="help">{{ help }}</span>
    {% endif %}
{% endblock %}
{% block form_row %}
{% spaceless %}
    <div>
        {{ form_label(form, label|default(null)) }}
        {{ form_errors(form) }}
        {{ form_widget(form) }}
        {{ block('field_help') }}
    </div>
{% endspaceless %}
{% endblock form_row %}
Note: this is copied from SimpleThings/FormExtraBundle/Resources/views/Form/field_type_help.html.twig with field_row changed to form_row
Then load this layout in your form Twig, with something like this at the top:
{% form_theme form 'custom_layout.twig' %}